The interactions of high density lipoproteins, apoA-I and apoA-II, with the phospholipid, lysolecithin, have been evaluated in order to obtain the affinity constants and structural changes in the proteins produced by the phospholipid. The rates of combination of the two subunits of human chorionic gonadotropin have been evaluated. The data are best described by a second order recombination and a first order conformational transition. The role of water in stabilizing the properties of proteins, nucleic acids and membranes has been reviewed.